1. Field of the Invention
The present invention relates to air bags which are one of safety devices of motor vehicles. More specifically, the present invention is intended to provide an uncoated woven fabric for air bags of motor vehicles, which has suitably adjusted air-permeability, and is particularly soft and light-weight in addition to excellent in compactness, by an economical production process with maintaining required mechanical properties. The present invention also relates to the production process and system of the uncoated woven fabric.
2. Disclosure of the Prior Art
As users' safety consciousness has risen, recently, the number of motor vehicles fitted with air bags as one of their safety parts has rapidly increased. Air bags serve as protection of users of motor vehicles including drivers and passengers against traffic accidents such as collisions. For example, upon a collision, a sensor perceives impact, thereby actuating an inflator to generate gas at high pressure and temperature, followed by rapid deployment of an air bag which has been folded and stored in an appropriate part of a motor vehicle such as a steering wheel, an instrument panel, a recess in the door frame, etc., to protect the user against the collision.
Heretofore, woven fabrics coated with synthetic rubber such as chloroprene, chlorosulfonated olefin, silicone and the like have been used for air bags from the viewpoints of heat resistance, air barrier properties (air-permeability) and fire and flame retardancy.
However, these coated fabrics have many drawbacks such as increase in weight and decrease in pliability of fabrics, increase in production costs, difficulty in recycling, and the like. At present, silicone-coated fabrics are still used and their drawbacks are considerably improved. However, they are still insufficient.
Then, nowadays, woven fabrics to which no coating is applied, i.e., uncoated woven fabrics, are mainly used for air bags. Uncoated woven fabrics for air bags are required to have low air permeability. For this purpose, their production processes are generally divided into the following two processes.
One process (Process No. 1) is to weave a fabric on a loom tightly so that the number of warp yarns per inch and the number of weft yarns per inch are as dense as possible. In this process, laps of yarns should be constructed mechanically to prevent air leakage. Then, there is a problem of productivity because the load of the loom becomes too heavy, which makes it difficult to increase the speed at which the fabric is woven. In addition, since warp tension should be increased so as to increase the number of yarns per inch, the warp yarns are liable to be damaged, which results in a problem of reliability of mechanical properties of the resultant woven fabric.
In the other process (Process No. 2), as disclosed in JP-A 4-281062, a high density woven fabric is produced by using yarn having hot-air shrinkage of 6 to 15% (dry heat treatment at 160.degree. C.), subjected to water-bath treatment at 60 to 160.degree. C. and then subjected to drying without heat fixing. Indeed, this process is advantageous over Process No. 1 because the problem of mechanical properties of a woven fabric of Process No. 1 is not present and high density weaving is not required. However, regarding the drying step, this JP-A 4-281062 merely describes that the drying step "is carried out at 130 to 170.degree. C. with a conventional machine" and it is silent on a particular kind of the dryer and drying conditions.
Moreover, in case of Process No. 1, for obtaining an uncoated woven fabric for air bags having low air-permeability, there are such problems as the decrease in productivity and damage of the warp yarns, i.e., less reliability of mechanical properties of the resultant fabric as described above.
In case of Process No. 2, when drying and finishing are carried out by a conventional dryer which is employed for a normal drying step, such as a cylinder dryer, shrink surfer dryer or a heat setter (tenter), insufficient pliability of a fabric after drying and wrinkling on the surface of a fabric due to rapid drying of the fabric are observed.
If a fabric lacks pliability, deployment of the air bag is not smooth, which results in rupture of a stress concentration part. Or, bad workability is caused in a step for turning an air bag inside out after sewing it, which requires much man-hour and is not desirable from the economical viewpoint. Moreover, regarding the above problem of wrinkling, when an air bag is deployed and stress concentration occurs at a wrinkled part, rupture of that wrinkled part of the air bag may be caused due to lowering of mechanical properties, for example, lowering of tear strength of that wrinkled part. This is a problem in view of protection of users.